Information on the in situ deformability of rock masses in the earth is of particular importance in determining the suitability of a site for construction of structures either on the rock or within the rock mass. Knowledge of the deformability of the deep rock masses is necessary to allow proper numerical modeling of the rock structures, to calculate the stresses in the rock from observed strains or deformations, and to properly determine the stresses experienced by a rock mass to enable an assessment of the stability of openings to be formed in the rock.
The underground earth masses are accessed for testing purposes through a hole drilled from the surface--a preexisting hole drilled for other purposes, as for oil and gas exploration, or one drilled specifically for the purpose of allowing measurements to be taken of the deep earth formations. A test device, which may be one of a variety of constructions, is lowered to the selected depth and is operated to apply pressure to the walls of the hole. The resulting deformation of the wall areas under pressure is measured and related to the applied pressure to estimate the deformability of the rock.
Examples of borehole displacement testing devices are the borehole jack devices shown in U.S. Pat. Nos. 3,446,062 and 3,961,524. These devices use pairs of shoes or bearing plates, formed as portions of a cylinder, which move inwardly and outwardly relative to one another. Hydraulic fluid under pressure is provided to pistons which drive the shoes apart against the walls of the borehole, and displacement sensors measure the distance that the shoes are displaced relative to one another after pressure is exerted by the shoes. The shoes may typically displace the rock a few hundredths to a few tenths of an inch under several thousand pounds per square inch of pressure. The displacement of the shoes and the applied pressure provide data that may be used to estimate characteristics of the rock, such as the modulus of elasticity.
In another test method, the CSM cell method, the radial displacements of all points on the borehole wall in response to hydrostatic loading are integrated to determine an aggregate volume change of the borehole. By calculations based on elastic theory, it is possible to calculate the modulus of rigidity of the material surrounding the borehole from a knowledge of the hydrostatic loading and the measured volume change.
The foregoing and other techniques for measuring or estimating the in situ deformability of rock masses generally do not offer reliable and accurate deformability values. Each method produces data, from which the deformability is estimated, which is widely scattered and has large standard deviations. Variations in the estimates of deformability as obtained by the different methods are notable. The primary reason for the discrepancies observed within measurements taken by a single method and between the various methods is the existence of discontinuities in the rock mass. These discontinuities affect the loading conditions, stress distributions, deformations, strains and other parameters used to determine the in situ deformability of the rock. Although such discontinuities can be modeled, and their effect on the in situ deformability can be estimated, the mapping of discontinuities, particularly those at some distance from the borehole, is difficult if not impossible.
As an illustration of the effect of discontinuities in the rock mass, it is observed that fissuring at the borehole wall surface allows the rock surrounding the borehole to be compressed more easily, giving a larger displacement under the applied pressure than would be found if the rock were continuous. In particular, the act of drilling the hole itself may cause disruptions in the borehole surface rock. A borehole jack will primarily compress the rock directly under the curved shoes of the jack, with most of the compressive strain in the rock extending only a short distance into the rock from the shoes. Thus, the surface discontinuities will have a strong influence on the compressibility of the rock as measured by the jack.
The borehole jack method also has other limitations which lead to inaccuracies in the resulting estimates of rock deformability. The semi-circular shoes which press against the walls may not perfectly match the curve of the borehole, resulting in much higher pressures applied at certain localized areas and little or no pressure at other areas. Even for a fairly smoothly bored hole, a shoe which has a 90.degree. cylindrical surface may have only 7.degree. to 17.degree. included angle of contact of its surface with the rock. In many cases the borehole itself may have irregularities or protuberances which are subjected to far higher pressures than the calculated average pressure applied by the shoe to the borehole wall. Depending upon the position at which the measurements of the displacements between the bearing shoes are taken, deformations of the bearing shoes themselves, e.g., a bending or "bowing" of the ends away from the middle, may introduce further errors into the displacement measurements.